Sedimentary Geology of the Middle Carboniferous of the Donbas Region

OPEN Sedimentary geology of the middle

SUBJECT AREAS: Carboniferous of the Donbas region GEOLOGY SEDIMENTOLOGY (Dniepr-Donets basin, Ukraine) Douwe J. J. van Hinsbergen1, Hemmo A. Abels1, Wolter Bosch1, Flora Boekhout2, Alexander Kitchka4, Received Maartje Hamers1, Douwe G. van der Meer3, Mark Geluk5 & Randell A. Stephenson6 8 September 2014

Accepted 1Department of Earth Sciences, University of Utrecht, Budapestlaan 4, 3584 CD Utrecht, the Netherlands, 2Institut fu¨r Geologie und 28 January 2015 Pala¨ontologie, Westfa¨lische Wilhelms-Universita¨t, Corrensstrasse 24, 48149 Mu¨nster, 3Nexen Petroleum UK Ltd., Charter Place, Vine Street, Uxbridge, Middlesex, UB8 1JG, United Kingdom, 4SE NaukaNaftogaz Res. Inst., NJSC Naftogaz of Ukraine, 8 Kyivska Published St., 08132 Vyshneve, Ukraine, 5Shell International Exploration and Production B.V., Kessler Park 1, 2288 GS Rijswijk, the 20 March 2015 Netherlands, 6School of Geosciences, Meston Building, King’s College, University of Aberdeen, Aberdeen AB24 3UE, UK.

Correspondence and The Paleozoic Dniepr-Donets Basin in Belarus, Ukraine, and Russia forms a major hydrocarbon province. Although well- and seismic data have established a 20 km thick stratigraphy, field-studies of its sediments requests for materials are scarce. The inverted Donbas segment (Ukraine) exposes the middle Carboniferous part of the basin’s should be addressed to stratigraphy. Here, we provide detailed sedimentological data from 13 sections that cover 1.5 of the total of D.J.J.H. (d.j.j. 5 km of the Bashkirian and Moscovian stages and assess the paleoenvironment and paleo-current [email protected]) directions. Middle Carboniferous deposition occurred in a shelf environment, with coal deposition, subordinate fluvial facies, and abundant lower and middle shoreface facies, comprising an intercalated package of potential source and reservoir rocks. Sedimentary facies indicate a paleodepth range from below storm wave base to near-coastal swamp environments. Sedimentation and subsidence were hence in pace, with subtle facies changes likely representing relative sea-level changes. Paleocurrent directions are remarkably consistently southeastward in time and space in the different sedimentary facies across the Donbas Fold Belt, illustrating a dominant sedimentary infill along the basin axis, with little basin margin influence. This suggests that the middle Carboniferous stratigraphy of the Dniepr-Donets basin to the northwest probably contains significant amounts of fluvial sandstones, important for assessing hydrocarbon reservoir potential.

he Dniepr-Donets Basin (DDB) forms a major hydrocarbon and coal basin in Eastern Europe with significant commercial significance1–7, forming a intra-cratonic, deep rift with up to 22 km of sediment1,8 that T underwent its main basin fill history between the late Devonian and Permian9–11. It is located between the Ukrainian Shield to the south and the Voronezh Massif to the north, in the southwest of the East European Craton (EEC; Fig. 1). The basin’s infill is poorly exposed, and knowledge about its sedimentary evolution largely relies on subsurface data. The eastern part of the basin, however, has been inverted in Permian and younger times and forms the Donbas Foldbelt, where Devonian and younger (volcano-) sedimentary rocks of the DDB are exposed12,13. The Donbas Foldbelt is characterised by WNW-ESE trending long-wavelength folds and faults14,15 (Fig. 1). As a result of Permian and/or Late Cretaceous/Paleogene inversion of the Donbas Foldbelt16–19, Carboniferous sediments are exposed. The middle Carboniferous is exposed within the axial zone of the Donbas Foldbelt as well as near the southern margin, where it overlies pre-and syn-rift Devonian to Lower Carboniferous sediments. Outcrops in the Donbas Foldbelt are mainly confined to road and river sections and quarries. We present detailed sedimentological descriptions and paleoenvironmental interpretations from 13 sections covering 1.5 km of a total of ,5 km Bashkirian and Moscovian stratigraphy. We briefly discuss the potential implications of our findings for the sedimentary geology of the DDB farther to the northwest, where exposures of the middle Carboniferous are absent.

Geological setting. The DDB overlies an Archean to Lower Proterozoic crystalline basement20,21 and trends NW- SE from Belarus through Ukraine to Russia, connecting with the Karpinsky Swell to the east (Fig. 1). The Donbas segment of the DDB formed at the southern margin of the EEC, which belonged to Laurussia in the Late Paleozoic

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Figure 1 | Geological map of the Donbas Foldbelt, and in the inset, the location of the Donbas in the regional East European structural framework (modified from Stovba and Stephenson31). and was located at near-equatorial latitudes during the Early Carbo- A post-rift sag sequence is bounded by pre-Carboniferous and niferous moving to ,15u northerly latitudes in the Permian22–25. pre-Triassic unconformities12. Following the first stages of rift react- Towards the southeast, the width and thickness of the basin fill, the ivation in the Visean, the Ukrainian shield was covered by a thin layer intensity of inversion-related deformation, the degree of metamorphism of upper Visean and younger sediments. A large volume of fluvial of its exposed sediments, as well as the degree of syn-rift volcanic clastic material was transported by river systems from the northwest activity increase26. Basin inversion occurred in two or three phases in along the basin axis, prograding into a deeper water basin that existed Permo-Triassic and Late Cretaceous-Early Cenozoic time16–19. since Devonian time39. Serpukhovian and younger Carboniferous Shortening of the Donbas Foldbelt may be Permian27–30 or Late sediments commonly consist of cyclothems of marine limestone or Cretaceous-Early Cenozoic in age31,32. Inversion occurred along shale at the bottom to coal and paleosol beds at the top and form the WNW-ESE rift-bounding faults, and formed a central ‘Main focus of this study40. Anticline’33 flanked by gentle folds. To the north, the Donbas Foldbelt During the Carboniferous and much of the Early Permian, the borders the Voronezh Massif (Fig. 1) along large thrusts and reverse DDB gradually subsided. The rate of subsidence was high; the thick- faults; in the south the contact of the Donbas Foldbelt with the ness of the Carboniferous-Lower Permian, dominantly clastic stra- Ukrainian Shield is formed by reverse faults8,32. tigraphy increases from 2–3 km in the northwest to about 11 km in Sedimentation in the DDB started in Middle Devonian time with the southeast of the basin. Increased aridity and relative sea level fall deposition of pre-rift sediments under platform conditions. Judging during the early Permian resulted in deposition of red beds, carbo- from the absence of marginal facies, and based on low-temperature nates, and evaporites16,41–44. Upper Permian sedimentary rocks are geochronology, that platform originally extended far beyond the absent and post-Permian deposits are fluvio-lacustrine to shallow present limits of the basin34,35. The main rifting phase that formed marine clastics and carbonates with a maximum thickness of ,2– the DDB started in the Late Devonian (370–363 Ma36,37 and was 2.5 km12. associated with basement doming and mafic to intermediate mag- The Carboniferous stratigraphy of the Donbas region studied here matism13. Devonian rifting led to widespread salt-deposition, has been divided into lithostratigraphical suites45. Four suites are reflected by local diapirs in the Donbas Foldbelt38. Salt formations recognized in the Mississippian, nine in the Early and Middle probably filled deep-water basins, and were preceded by deposition Pennsylvanian (or Bashkirian and Moscovian; Fig. 2). The ‘middle’ of organic-rich anoxic shales and carbonates2. Uppermost Devonian Carboniferous is more than 5 km thick (,3 km Bashkirian and 2– rocks formed large sub-aqueous clastic fans along the southern basin 2.5 km of Moscovian12,46,47). Limestone beds form regional markers margin and containing shales interbedded in coarser clastics. A total in the successive suites and have been labelled with a letter from the thickness of 4–5 km of the syn-rift basin infill was estimated2. Latin alphabet accordingly. Smaller or regionally less coherent lime-

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stone beds within suites were assigned subscript successive numbers (Fig. 2). Additional subdivision has been made on the basis of the fossil content of the succession. Biozones have been coded differently from the suites, and the boundaries do not all correspond with the suite boundaries (Fig. 2). Biostratigraphy and suites subdivision form the basis for correlation of the Donbas stratigraphy to the Western European and American stratigraphies48.

Results Thirteen sections in the Donbas region were studied in detail (Figs. 1, 2, 3), covering over 1.5 of the total of ,5 km of middle Carboniferous stratigraphy (Fig. 2). The studied sections together represent the majority of available outcrops of Bashkirian and Moscovian stratigraphy in the area. Extensive descriptions of each section are given in the Supplementary Material, with detailed sedimentary logs, descriptions, field photographs and section locations. Here, we briefly describe the sections in stratigraphic order. The oldest part of the stratigraphy was studied in the 65 m thick Stepano-Krynka section (lower Bashkirian), dominated by sandstone with subordinate clay and silt and an occasional coal bed. Most sandstones in the section are well-sorted middle sand with abundant current-induced cross-bedding. It contains intervals with tree trunk prints (Fig. 4B). One, relatively thin sandstone (SK.2A) is present that is much finer grained and contains hummocky cross stratification. Paleocurrents were east to southeast. The Chegharniki section is early Bashkirian in age, and covers 118 m. The section has 39% sandstone content, with the rest shales. In the middle part of the section, sandstones are fine and well sorted (Fig. 4A), and occasionally show hummocky cross-stratification (Fig. 4D). The basal sandstone shows current-induced cross beds. The topmost thick sandstone is coarser-grained, shows larger scale foresets, and some intervals with very poor sorting. In the section plant remains are common. Sparse paleocurrent measurements suggest an eastward paleocurrent direction. Section Illyria is early Bashkirian in age and is characterized by a relative high amount (73%) of poorly exposed shales and silts. The top of the section is characterised by fine to middle well-sorted sand with large scale swaley cross stratification. The upper Bashkirian Bulavinskoye section exposes well-sorted fine sandstone units that contain hummocky cross-stratification or current-induced large-scale cross-beds (set height ,50 cm). Paleocurrent directions in these sandstones are consistently north- east. A few sandstones in the section are coarser grained and occasionally contain tree trunk prints, poor sorting, and massive bedding. This unit also shows some trough-like structures. Limestone beds are present in the middle of shaly intervals. One coal interval has been observed. Below the studied section, a thick interval of shales (,200 m) is present, overlying a thick sand-rich interval (,200 m), containing coarse-grained fluvial sandstones. The sandstones from the upper Bashkirian Yur’ivka section are generally well sorted, display plane bed lamination, are fine to middle grained, and show hummocky cross stratification. The uppermost sandstone contains large-scale current-induced cross- bedding. Thick limestones are present within shale-dominated intervals. The stratigraphy of the upper Bashkirian Orlovo-Ivanivka section is dominated by thick shale and silt intervals with well-sorted, very- fine to fine-grained sandstones at the top, and coarser sand intervals at the base. Cross-beds indicate a consistent east to southeast paleocurrent direction. Figure 2 | Middle Carboniferous stratigraphy showing the The lower part of the upper Bashkirian-lower Moscovian lithostratigraphic position of the sections studied in this paper in context Toshkovka section is characterised by coarse-grained, poorly sorted, eastern European stages, regional suites, marker limestones, and sometimes massive, tree trunk print-bearing sandstone units. Some biozones of the Bashkirian and Moscovian stages3. Rose diagrams in large-scale trough cross stratification is present. Above this interval, a bottom panel indicate paleocurrent directions per sandstone type showing thick section of shale contains a single thick limestone bed and is a general E-SE paleocurrent, i.e. parallel to the basin axis. overlain by fine-grained sandstones. Paleocurrent directions indicate

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Figure 3 | Sedimentary logs of the sections studied from middle Carboniferous stratigraphy of the Donbas region. Locations of the sections are indicated on Figure 1. See online appendix for detailed descriptions, field photographs, and extensive documentation of all sections. dominant paleo-flow to the southeast. The section is topped by a These units are fining upwards, with massive intervals in the base thick limestone bed, overlying shales with coal layers. and better-developed current-induced crossbeds towards the top. The upper Bashkirian Stepnoye section has a low amount of sand The average paleocurrent direction towards the southeast, with and is dominated by poorly exposed shales. The sandstones in the almost 180 degrees variability. section consist mainly of moderately to well-sorted, middle to fine The upper Moscovian Pervomaysk sandstone is coarsening sandstones that display small to large-scale current-induced cross- upwards towards the middle, and fining upwards towards the top bedding (Fig. 4F). Paleocurrent measurements are south-eastward of the unit. The lower fine to middle sand part is characterized by with ,120u of variation. current induced cross-bedding (Fig. 4C). The middle part is much The lower Moscovian Zolotoye sandstone can be divided into two coarser grained and less well sorted. Large trough structures and tree parts that show different sedimentary characteristics. The lower part trunk prints were observed. The top part is finer-grained and con- shows poorly sorted, bedded, massive coarse sandstone. The upper tains abundant mega- and intermediate scale current-induced cross- part consists of better-sorted coarse sandstone with large-scale cur- bedding. Three measured paleocurrent directions suggest SE paleoflow. The sandstone overlies a shale-dominated stratigraphy rent-induced cross-bedding. with a limestone bed and immediately below the Pervomaysk sand- The lower Moscovian Malo-Orlovka section is contains a high stone a transition interval of sandy silts occurs. amount of shales and siltstones with brachiopod-rich beds The upper Moscovian Illinka sandstone is very coarse and poorly- (Fig. 4E), and subordinate sandstone. Only three sandstone units sorted, but shows well-developed current-induced large-scale cross- are present that have a grainsize larger than very fine sand to silt. bedding. The basal and top parts of the sandstone show slightly better The upper Moscovian Fashchivka section is characterized by sorting and finer material. Towards the top, trough cross-beds occur. prominent sandstone units intercalated in shales and silts. Half of Paleocurrent measurements show consistent ENE paleoflow with these units are well sorted, fine to middle sandstones with current- ,90u of variation. Above the sandstone, a shale interval is exposed, induced intermediate scale (,25 cm) crossbeds and some indica- with one thick limestone interlayer. tions for hummocky cross-stratification. The basal and top unit consists of poorly sorted coarse, locally massive sandstones with tree Generalised lithological characteristics and interpretation. The trunk prints. Several intervals contain large-scale current-induced middle Carboniferous stratigraphy in the Donbas region is cross-beds (,50–100 cm) with variable paleocurrent directions. characterized by a large amount of (poorly exposed) shales and

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Figure 4 | Field photographs of various characteristic lithologies (photos by H.A. Abels and W. Bosch). (A) wavey laminated very fine sand and siltstone, Chegharniki section; (B) Tree trunk prints in sandstones, Stepano-Krinka section; (C) Large-scale cross-bedding, Pervomaisk section; (D) Hummocky cross stratification, Chegharniki section; (E) Brachiopods in fine sandstones, Malo-Orlovka section; (F) Cross-bedded sandstones, Stepnoye section. siltstones with intercalations of sandstone bodies and some relatively tigraphy of the Donbas region50,51. The coal intervals in the Donbas thin limestone beds. Shales have a slaty character but no newly grown region are usually related to continental low-moor depositional mica has been observed showing sub-lower greenschist facies environments during transgressions52 and have a wide extent with metamorphism. Hereafter, a generalized sedimentary description some seams covering the entire basin50. of the middle Carboniferous lithologies is given. Clay, silt, and limestones intercalate with sandstones in the middle Limestone beds, generally up to several meters thick, form region- Carboniferous of the Donbas region. We identify four sandstone ally well-traceable, basin-wide markers used for lithostratigraphic facies according to their sedimentological characteristics. In the sedi- correlation in the Donbas. They are the only lithology with marine mentary logs of Fig. 3, these interpreted classes are indicated in a fossils46,49. The limestones in the middle Carboniferous of the Donbas separate column. Sandstones of different facies often occur in stacked area intercalate with shale intervals. Limestones are mainly black sequences. Based on visual inspection with a hand lens, the minera- mud- to wackestones. Some contain macroscopically recognisable logical composition of the sandstones is generally ,40% quartz, fossils, such as crinoids and brachiopods. Limestone beds are typ- ,25% feldspar, ,25% rock fragments, heavy minerals and detrital ically 0.5–1.5 m thick. We found rare shell prints in the abundant mica. Grains have mostly sub-angular and high spherical shapes. claystones that are interbedded with the various sandstone facies, but Hereafter, sedimentary characteristics per distinguished group (A did not encounter calcareous fossils or ichnofossils, although these to D) are given. have been reported12. Within the claystones, thin intervals (,50 cm) Sandstones of Group A include (very) coarse, poorly sorted sand- occur with high organic carbon content, in which some small plant stones, which often contain tree trunk prints and other large plant remains have been found, occasionally up to anthracite grade. These remains. Massive beds are common although occasionally upper represent periods of relative sea-level lowstand, with probably near- plane bed, large-scale trough structures, and well-graded current- shore swamp formation. The sedimentary facies suggest that depos- induced trough cross-bedding occur. Grainsizes vary from fine sand ition of the middle Carboniferous stratigraphy occurred in a narrow to small pebbles of around 2–3 cm. Grading from fine sand to pebble- bathymetry range, from around the fair-weather wave base to coastal sized grains occurs but non-graded intervals are more prominent. swamp areas. The abundant presence of coal layers within the shale The thickness of Group A sandstone intervals mainly varies between and silt intervals illustrates the flatness and shallowness of the basin. 1 to 5 m and they often occur in the lower half of a thick sandstone Anthracite intervals are abundant in the middle Carboniferous stra- body where they are topped by sandstones of groups B and C (Fig. 3).

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Some of the very coarse intervals appear to only consist of quartz alternate with layers of silt and locally shales and rarely contain plant grains, but also intervals relatively rich in feldspar occur. Sandstones remains. Units of this group are generally laterally very continuous at of Group A are least abundant of the four types of sandstones and outcrop scale. Sandstone units form the transitional lithology only few paleocurrent measurements were collected, which indicate between intervals of shale and sandstone beds of other groups. variable, but generally eastward paleoflow directions (Fig. 2). The fine sandstones in this group are interpreted as quieter marine The massive bedding, poor sorting, coarse grain size up to pebble- deposits that lack evidence for proximity of a river mouth as sug- size, and large-scale trough structures are interpreted to reflect mass gested by the previously described groups. The sandstones are well flow deposition. The frequent occurrence of prints of transported sorted indicating some distance between source area and site of tree trunks suggests a terrestrial or near-shore environment. Hence, deposition. The common occurrence of hummocky cross-stratifica- we suggest that these beds originate from short-lived high-energy tion and lower plane bed together with intercalation with (and trans- fluvial pulses with high sedimentation rate preventing development ition to) mudstones suggests they are the most distal, lowest energy of channel systems including point-bar sequences. Erosional surfaces sandstone deposits observed in the stratigraphy. The occasional rip- have not been observed, which may indicate short-lived high runoff ple cross-lamination in silts suggest sedimentation in water depths and sediment supply events that dumped coarse to pebbly sediments still above the storm wave base, but the absence of drapes on foresets in the basin. The presence of immature sandstones, especially in the and clear separation of sets that would suggest tidal influence leads us north, may indicate proximity to the source area. The preservation of to interpret the depositional depth as below the fair-weather wave the large tree trunk remnants shows low oxygen content at the site of base54,55. This facies is therefore ascribed to lower shoreface deposition due to high sedimentation rate. Beds of Group A are environments53. frequently followed by beds of Group B which contain sedimentological indications for tidal influence, and we suggest that group A Discussion sandstones reflect river mouth or proximal deltaic mass-deposition We report field observations and interpretations from the sediment- events in times of high run-off. ary geology of the middle Carboniferous stratigraphy of the Donbas Sandstones in Group B consist of middle to coarse-grained, mod- Foldbelt, providing new environmental information in addition to erately-sorted sands with current-induced cross beds. Some large the basin reconstructions based on previous field observations, and foresets are up to 1 m thick, but most sets are around 20 to 50 cm mostly borehole and seismic data3,12,49,56–59. Analysis of the sediment- high. Set boundaries tend to be parallel to the dominant layering and ary characteristics of the sandstone successions in the Donbas are continuous, especially when large Few tree trunk prints and some Foldbelt, the best exposed lithology, shows that the depositional plant remains occur. Paleocurrents are eastward (Fig. 2). environment varied between the terrestrial, low-energy coal envir- The mainly horizontal set boundaries and set thicknesses of up to onments and storm-wave base paleobathymetries: We interpret the 1 m observed in these sandstones are interpreted to reflect high four different groups of sandstones described above as river mouth to sediment supply and sufficient accommodation space. The lack of deltaic environments, with water depths generally confined to the erosional surfaces and trough structures, as well as the finer sediment upper tens of meters, above the storm wave-base. Fine-grained sedi- drapes on the set boundaries suggest tidal influence. Group B sand- ments including clay and silt intercalated with the most distal sandstones frequently follow on Group A sandstones and are hence inter- stone facies, our Group D, still provide evidence for some wave action preted as the lower energy continuation of the mass-flow deposits in during deposition, indicating that the storm wave-base can generally a river mouth bar or upper delta regime, in shallow (marine) waters. be regarded as the deepest marine facies observed. We found no clear evidence for barrier island systems or long-shore The apparent continuity of the sheet-like sandstone bodies, the bars, suggesting a river-dominated deltaic clastic regime. clear wave-generated sedimentary characteristics without showing Sandstones of group C consist of fine to middle-grained, well- beach deposits, the absence of erosional (sub-marine) channel sys- sorted sandstones with current-induced small to intermediate scale tems and turbidites in studied sections, and the correlation potential (,15–30 cm) cross-beds. Some hummocky cross-stratification is of the thin limestones and coals all indicate the basin has been a very present especially in the basal and top parts of the units where these low gradient and shallow epi-continental shelf sea. The interpreta- structures intercalate with current-induced tabular cross-beds, with tion of the sandstones as fluvial, delta-front mouthbar, upper, middle minor trough cross-stratification The sandstones of group C are and lower shoreface, and shelf sediments does however not clarify the generally 4 to 10 m thick. Some thin layers of finer sediments inter- specific absolute coastal profile that was present in the basin. The calate, mainly silts. Plant remains, mostly leaves and small branches, depth of fair-weather wave base commonly lies at approximately 5 to occur. Individual foresets are often well separated by films of finer 15 m60, so shelf deposition was possibly just slightly deeper. Our sediment. We binned the paleocurrent directions measured in facies interpretation suggests that throughout the middle Groups C and D, showing generally SE-ward paleoflow (Fig. 2). Carboniferous, water depths throughout the Donbas region rarely Sandstones in this group lack large-scale continuous current- exceeded 20–30 m. Throughout most of its exposed stratigraphy, the induced cross-beds and are interpreted to result from lower or more basin was in such relatively deep conditions for most of the time, and continuous sediment supply and weaker currents. The occasional only short intervals of shallower conditions occurred, indicated by presence of hummocky cross-stratification in the basal and top parts the deposition of sandstone types A and B, and the absence of paleo- of the sandstone unit in this group suggests open water environ- sols, rooted vegetation, and clear erosion surfaces. ments. We therefore ascribe sandstones in this group to a lower to The clean, carbonate-poor, non-bioturbated sandstones contain- middle shoreface environment53. Some influence of tides may be ing tree trunks suggest high sedimentation rates in the basin54. The represented by clear separation of individual foresets. Hence, lack of erosional levels suggests that the succession is fairly continu- Group C sandstones may either result from decreasing energy during ous, as does the absence of evidence for paleosols. During the deposition following deposition of Group B sandstones, or a more ,12 Myr of middle Carboniferous time, approximately 5 km of distal equivalent of Group B. sediment was deposited, averaging to ,40 cm/kyr, in line with the Sandstones of Group D are very-fine, to fine sands that are well sedimentary facies that indicate rapid dumping of clastics in a rapidly sorted. Hummocky cross-stratification is common, as well as lower subsiding basin. plane bed horizontal stratification and some small-scale wavy lam- The narrow paleobathymetry range and the absence of evidence ination. Sandstones with these characteristics are normally 10 cm, for large hiatuses in the studied sections show that rapid subsidence but occasionally up to a few meters thick. Locally, they form well- and sedimentation kept pace throughout the middle Carboniferous bedded units, with beds of around 10 to 50 cm. At many places they in the Donbas. Moreover, the absence of syn-sedimentary deforma-

SCIENTIFIC REPORTS | 5 : 9099 | DOI: 10.1038/srep09099 6 www.nature.com/scientificreports tional features, internal angular unconformities and major mass- 5) The coals, sands and shales present in the Donbas Foldbelt, flow deposits – other than those of Group A sandstones, which are comprise an intercalated stacked package of potential source interpreted as rapidly dumped sandstones during high run-off rocks, reservoir sands and sealing lithologies, which likely con- events, suggest that subsidence was fast but gradual, in line with tinue towards the northwest into the Dniepr-Donets basin earlier interpretations that the middle Carboniferous was a time of (DBB), one of the major hydrocarbon provinces of Europe. thermo-tectonic subsidence of the DDB12,40. Our consistent SE-ward paleocurrent measurements across The regionally and temporally very consistent SE-directed paleo- the middle Carboniferous of the Donbas Foldbelt suggests that current measurements (Fig. 2) also attest to this interpretation: sedi- the time-equivalent deposits at depth further to the northwest ment transportation was dominantly along-axis throughout the in the DDB are probably dominated by fluvial, sand-rich middle Carboniferous without clear local perturbations. These new deposits, relevant for the assessment of reservoir potential. data are more or less in line with the few published data on paleocurrent directions, which showed paleoflow to the south and southeast49. Methods The margins of the DDB in the Donbas in the middle Results in this paper were obtained through field observation of sedimentary rocks Carboniferous must have been further to the north and south than using a magnetic compass, a hammer, a hand lense, a centimetre, a notebook, a pencil, and a digital camera. currently outcropping, and were probably of low topography: basin inversion phases after the Carboniferous resulted in some shortening of the north-south extent of the Donbas Foldbelt31,32 and likely ero- 1. Chekunov, A. V., Kaluzhnaya, L. T. & Ryabchun, L. I. The Dniepr-Donets sion of postrift sediments at the rift shoulders, and the sedimentary palaeorift, Ukraine, deep structures and hydrocarbon accumulations. J Petrol Geol characteristics and paleoflow directions do not show signs for basin 16, 183–196 (1993). 2. Ulmishek, G. F. Petroleum Geology and Resouces of the Dnieper-Donets Basin, marginal facies. The consistent along-axis paleocurrent directions Ukraine and Russia. USGS Bull 2201-E, 1–14 (2001). indicating southeastward paleo-flow, in combination with shallow- 3. Sachsenhofer, R. F. et al. The Donets Basin (Ukraine/Russia): coalification and marine to paralic paleoenvironments suggest that further to the west, thermal history. Int J Coal Geol 49, 33–55 (2002). where the middle Carboniferous cannot be found in outcrop, similar, 4. Privalov, V. A., Sachsenhofer, R. F., Panova, E. A. & Antsiferov, V. A. Coal Geology of the Donets Basin (Ukraine/Russia): an overview. Berg Hu¨ttenma¨nn or fluvial, sand dominated stratigraphies can be expected. Our results Monatsh 149, 212–222 (2004). suggest that the middle Carboniferous to the northwest of the 5. Zaitseva, L., Ivanova, A. & Zhernova, H. Study of the coal facies in Eastern Donbas region in the DDB thus likely contains abundant, more Ukraine. Int J Coal Geol 58, 75–85 (2004). proximal fluvial sandstones, which may be important for the res- 6. Stephenson, R. A. et al. Late Palaeozoic intra- and pericratonic basins on the East ervoir potential in this part of the stratigraphy. European Craton and its margins. Geol Soc London Mem 32, 463–479 (2006). 7. Alsaab, D., Elie, M., Izart, A., Sachsenhofer, R. F. & Privalov, V. A. Predicting methane accumulations generated from humic Carboniferous coals in the Conclusions Donbas Foldbelt (Ukraine). AAPG Bull 92, 1029–1053 (2008). We present a field study of the sedimentology of the middle 8. Stovba, S. M., Stephenson, R. A. & Kivshik, M. Structural features and evolution of the Dniepr-Donets Basin, Ukraine, from regional seismic reflection profiles. Carboniferous stratigraphy in the Donbas Foldbelt, eastern Tectonophys 268, 127–147 (1996). Ukraine. Our conclusions are: 9. Lyngsie, S. B., Thybo, H. & Lang, R. Rifting and lower crustal reflectivity: A case study of the intracratonic Dniepr-Donets rift zone, Ukraine. J Geophys Res 112, 1) Deposition in the Donbas occurred throughout the middle B12402–doi:10.1029–2006JB004795 (2007). Carboniferous in a shelf setting, with water depths rarely dee- 10. Stephenson, R., Egholm, D. L., Nielsen, S. B. & Stovba, S. M. Role of thermal per than the storm wave-base. The deposition of coal amidst refraction in localizing intraplate deformation in southeastern Ukraine. Nature clay and siltstone, combined with the subordinate abundance Geosci 2, 290–293 (2009). 11. Carpentier, S. F. A., Roy-Chowdhury, K., Stephenson, R. A. & Stovba, S. M. of major fluvial environments or erosional unconformities Delineating tectonic units beneath the Donbas Fold Belt using scale lengths suggest that the environment became terrestrial at the coastal estimated from DOBRE 2000/2001 deep reflection data. J Geophys Res 114, level, without regressions leading to significant erosion of prev- B10315–, doi:10.1029–2008JB006124 (2010). iously deposited sediments. All other sedimentary facies fall in 12. Aizenverg, D. E. et al. Field excursion guidebook for the Donets Basin. (Ministry of paleobathymetry ranges between these extremes, with sand- Geology of the Ukr.SSR, 1975). 13. Wilson, M. & Lyashkevich, Z. M. Magmatism and the geodynamics of rifting of stone intercalations that were deposited mainly on the middle the Pripyat-Dnieper-Donets rift, East European Platform. Tectonophys 268, to lower shoreface and sometimes in a fluvial mouthbar sys- 65–81 (1996). tem. 14. Sollogub, V. B., Borodulin, M. I. & Chekunov, A. V. Deep structure of the Donbass 2) Sedimentation throughout the middle Carboniferous of the and adjacent regions. Geol J 37, 20–30 (1977). , 15. Maystrenko, Y. P. et al. Crustal-scale pop-up structure in cratonic lithosphere: Donbas kept pace with subsidence at high rates of 40 cm/ DOBRE deep seismic reflection study of the Donbas fold belt, Ukraine. Geology kyr averaged over the entire ,12 Myr time span represented 31, 733–736 (2003). by the stratigraphy. The absence of evidence for major syn- 16. Stovba, S. M. & Stephenson, R. A. Style and timing of salt tectonics in the Dniepr- sedimentary faulting within our sections and the very consist- Donets Basin (Ukraine): implications for triggering and driving mechanisms of ent paleocurrent directions without major local deflections are salt movement in sedimentary basins. Mar Petrol Geol 19, 1169–1189 (2003). 17. Goncharenko, B. D., Keller, M. B. & Makhiarinsky, A. Y. Regional unconformities in line with previous interpretations that subsidence and cre- and petroleum productivity (in Russian). Geologiya Nefti i Gaza 4, 11–17 (1984). ation of accommodation space was dominated by thermo-tec- 18. Yevdoshchuk, N. I., Kabyshev, B. P., Prigarina, T. M., Chuprynin, D. I. & tonic subsidence of the DDB in a post-rift setting. Shevyakova, Z. P. Regularities in distribution and prediction of significant oil and 3) The facies changes recorded in our sections are probably con- gas accumulations in the Dnieper-Donets basin (in Russian). Naukova Dumka trolled by relative sea-level changes on the order of tens of 206, (1988). 19. Spiegel, C., Sachsenhofer, R. F., Privalov, V. A., Zhykalyak, M. V. & Panova, E. A. meters superimposed on the continuous thermo-tectonic sub- Thermotectonic evolution of the Ukrainian Donbas Foldbelt: evidence from sidence trend. zircon and apatite fission track data. Tectonophys 383, 193–215 (2004). 4) A regionally and temporally very consistent paleoflow dir- 20. Gorbatchev, R. & Bogdanova, S. Frontiers in the Baltic Shield. Precam Res 64, 3–21 ection throughout the middle Carboniferous in all different (1993). sandstone types suggests paleo-currents dominated by along- 21. Yegorova, T. P., Stephenson, R. A., Kozlenko, V. G., Starostenko, V. I. & Legostaeva, O. V. 3-D gravity analysis of the Dniepr-Donets Basin and Donbas axis infill of an identical northwest to southeast basin config- Foldbelt, Ukraine. Tectonophys 313, 41–58 (1999). uration as seen nowadays, with minor influence of infill from 22. Torsvik, T. H. et al. Phanerozoic polar wander, palaeogeography and dynamics. the basin margins. Earth-Sci Rev 114, 325–368 (2012).

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A new geodynamical–thermal model of rift evolution, with The authors would like to thank Poppe de Boer for comments and corrections, Cor application to the Dnieper–Donets Basin, Ukraine. Tectonophys 313, 29–40 Langereis for discussions, and Constantin Pechnikov for his field assistance. We thank Shell (1999). International Exploration and Production, Rijswijk, the Netherlands, and Shell Ukraine, 38. Kurilyuk, L. V., Vakarchuk, G. I., Slobodyan, V. P. & Khmel, F. F. Paleozoic especially Calum Macdonald and Maxim Vityk, as well as Sergiy Stovba from SGE evaporite formations of the Dnieper-Donets basin (in Russian). Sovetskaya NaukaNaftoGaz, Ukraine (now at SPK Geoservice Ltd) for logistical and financial support Geologiya, 4, 15–21 (1991). and discussion. We appreciate logistical support of the Institute of Geophysics NASU. 39. Gavrish, V. K. Depth structure and evolution of the Pripyat-Dniepr-Donets and DJJvH acknowledges the ERC for Starting Grant numbers 306810 (SINK) is supported Kenyan rift (in Russian). Geol J 45, 10–18 (1985). through NWO Vidi grant. HAA acknowledges an NWO Veni grant (863.11.006). 40. Dvorjanin, E. S. et al. Sedimentary cycles and paleogeography of the Dnieper Donets Basin during the late Visean-Serpukhovian based on multiscale analysis of well logs. Tectonophys 268, 169–187 (1996). Author contributions 41. Chirvinskaya, M. V. & Sollogub, V. B. Deep Structure of the Dniepr–Donets D.J.J.v.H., D.G.M., M.G. and R.A.S. designed the research. D.J.J.v.H., H.A., W.B., F.B., A.K., Aulacogen from Geophysical Data (in Russian). (Naukova Dumka, 1980). M.H., D.G.v.d.M., M.G. and R.A.S. performed field research. D.J.J.v.H., H.A.A., F.B. and 42. Kivshik, M. K., Oleksyuk, N. K. & Gladchenko, Y. A. Application of seismic A.K. wrote the manuscript. D.J.J.v.H., H.A.A., W.B. and F.B. designed the figures. H.A.A. exploration on the stage of Shebelinka gas field operation (in Russian). Oil Gas Ind and W.B. wrote the online appendix. All authors reviewed the manuscript. 1, 7–10 (1991). 43. Kivshik, M. K., Stovba, S. M. & Turchanenko, M. T. Certain features of the Additional information Dniepr–Donets depression structure from the regional seismic-stratigraphic Supplementary information accompanies this paper at http://www.nature.com/ investigations data (in Russian). Geol J (Kiev) 2, 87–98 (1993). scientificreports 44. Stovba, S. M., Maystrenko, Y. P., Stephenson, R. A. & Kusznir, N. J. The formation of the south-eastern part of the Dniepr-Donets Basin: 2-D forward and reverse Competing financial interests: The costs of the field research, six months of salary of modelling taking into account post-rift redeposition of syn-rift salt. Sed Geol 156, H.A.A. and F.B. and a consulting fee for D.J.J.v.H., A.K. and R.A.S. within context of the 11–33 (2003). research reported here, and conducted in 2006, were provided by Shell Exploration and 45. Popov, V. S. The Geological map of pre-Mesozoic sediments of the Ukrainian part Production, Rijswijk, the Netherlands, who at that time employed D.G.M. and M.G. of the Great Donbas. Kiev, Ministry of Geology of the USSR, scale 15500,000, 1 map How to cite this article: van Hinsbergen, D.J.J. et al. Sedimentary geology of the middle sheet (in Russian) (1965). Carboniferous of the Donbas region (Dniepr-Donets basin, Ukraine). Sci. Rep. 5, 9099; 46. Izart, A. et al. Stratigraphy and sequence of the Moscovian in the Donets basin. DOI:10.1038/srep09099 (2015). Tectonophys 268, 189–209 (1996). 47. Levenshtein, M. L., Sprinina, O. I., Nosova, K. B. & Dedov, V. S. Map of coal This work is licensed under a Creative Commons Attribution 4.0 International metamorphism in the Donetsk Basin (Paleozoic surface) at 1/500000: Kiev, License. The images or other third party material in this article are included in the Ministry of Geology of the USSR, 7 map sheets (in Russian). (1991). article’s Creative Commons license, unless indicated otherwise in the credit line; if 48. Izart, A. & Vachard, D. Subsidence tectonique, eustatisme et controle des the material is not included under the Creative Commons license, users will need sequences dans les bassins namuriens et westphaliens de l‘Europe de l’ouest, de la to obtain permission from the license holder in order to reproduce the material. To CEI et des USA. Bull Soc Geol Fr 165, 499–514 (1994). view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

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